Thin film multilayer reflectors suitable for reflecting visible or near-visible light are known. Such reflectors have long been made by evaporating thin films of different inorganic dielectric light-transmissive materials in succession on a glass substrate in a vacuum chamber. The different refractive indices of adjacent layers form tens or hundreds of interfaces, each of which reflects light by Fresnel reflection, and coherent constructive or destructive interference of reflected light components provides the reflector with its reflection and transmission properties. It is also know to make thin film multilayer reflectors by an extrusion process in which a multitude of alternating light-transmissive polymer materials are coextruded through a die, optionally passed through one or more layer multipliers, then cast onto a casting wheel or surface, and subsequently uniaxially or biaxially stretched. Such a technique can be used to make all-polymeric thin film reflective optical bodies, such as reflective polarizing films and reflective mirror films. See, for example, U.S. Pat. No. 5,486,949 (Schrenk et al.); U.S. Pat. No. 5,882,774 (Jonza et al.); U.S. Pat. No. 6,531,230 (Weber et al.); and U.S. Pat. No. 6,827,886 (Neavin et al.). In contrast to vacuum-coated inorganic dielectric thin film stacks, the multilayer reflectors made by polymer coextrusion techniques do not require a separate substrate for formation or handling.
Because of their reliance on coherent constructive or destructive interference of light from neighboring layer interfaces, and because such constructive or destructive interference is a strong function of the individual layer thicknesses, as well as other geometric factors, great care is typically needed to ensure that the layers are controlled to within a narrow tolerance of a design goal to ensure proper operation of the thin film interference device. As the physical size of the thin film device increases—such as for polymeric thin film reflective polarizers or broadband mirrors used in LCD devices, where the demand for larger screen sizes continues to grow—the need for such layer control can be even more important. Increasing the physical size of thin film reflective polarizers and mirrors that are manufactured in the form of thin, flexible all polymeric sheets or films also magnifies potential mechanical problems such as wrinkling, warping, and delamination.
Certain “thick film” multilayer reflectors are also known. These reflectors, which are generally associated with incoherent light reflection, are variously defined in the literature, for example, structures whose individual layers have an optical thickness of at least 0.45 micrometers, or structures whose individual layers have an average optical thickness of at least 5/4 times the average wavelength of light to be reflected. In any case, principles of incoherent light reflection inform the skilled artisan that, for most practical situations, thick film multilayer reflectors have peak reflectivities well below those achievable by their thin film counterparts, and thus the former are often considered inferior to the latter. Further, several references suggest that thick film multilayer stacks provide incoherent light reflection regardless of how thick the individual layers are, and that the layer thicknesses of a thick film multilayer stack have substantially no effect on such a stack's reflectivity.